Another thing to consider is the maximum delay you'll need. This is determined by measuring the distance from the MLP to the most distant and closest subs and taking the difference. For example:
Most distant sub = 14 feet away from MLP
Closest sub = 4 feet away from MLP
Difference in distance = 10 feet
The delay needed is computed from this distance difference and the speed of sound, which is 1126 feet/second. So in this case, the delay needed is:
delay = (10 feet) / (1126 feet/sec) = 0.00888 seconds = 8.88 msec
The regular 2x4 miniDSP units have a maximum of 7.5 msec delay, so you wouldn't be able to get optimum performance in this case. The safest bet would be the 2x4 HD version. The HD version has a huge 80 msec maximum delay and RCA connectors with a maximum output voltage of 2 Volts RMS, so it will drive just about any power amp to full power. It is a safe bet for any application with 4 subs or less.
If the 7.5 msec maximum delay of the normal 2x4 is enough for you, you could get the balanced 2x4. The unbalanced 2x4 (non-HD version) does not have enough output voltage to drive many pro amps to full power, but the balanced one does.